This invention relates to a method for recovering halogen from a carbonaceous material such as carbonaceous lining material used in electrolytic reduction cells, e.g., such as for the electrolytic production of aluminum from alumina dissolved in cryolite.
Electrolytic reduction cells for producing aluminum from alumina dissolved in cryolite require a container for holding the bath of molten salts. The containers for holding the molten salts typically are lined with a carbonaceous material to protect the steel of the container walls. The carbon lining is composed of a purified coke and carbonaceous binders such as tar or pitch in a layer of substantial thickness, e.g., such as 10 to 20 inches. The carbon lining can be surrounding by an outer layer of insulation such as insulating brick. However, insulation is not impervious to the transfer of molten salt ions from the electrolytic bath into the carbon lining.
Aluminum production in the electrolytic cell requires temperatures over 900.degree. C. for extended periods of time. It is well known that in the course of extended operation of the aluminum reduction cell, significant amounts of molten salts including fluorides are absorbed into the carbon liner. The cell eventually becomes cracked and distorted and must be taken out of operation. The cell is shut down and the deteriorated contents of the cell including the carbon liner and insulation are removed.
The spent carbon lining represents a valuable resource for the fluorine which has been absorbed from the fused cryolite bath into the carbon lining over the extended smelting performed in the cell.
Cryolite recovery plants have been operated for leaching cryolite from the spent carbon lining using caustic solutions. Caustic solutions can be drawn from an alumina refinery, and waste liquor returned to it.
An alternative and separate process for removing fluoride from carbon includes dissolving the fluoride in a sulfuric acid bath applied to the carbon lining.
It is an object of the present invention to increase the amount of fluoride extracted from the carbonaceous material of spent cell linings used in the electrolytic smelting of aluminum.
It is another object of the present invention to recover fluoride which could be used in subsequent chemical processes, e.g., as an AlF.sub.3 feed to an aluminum reduction cell.
These and other objects of the present invention will become apparent from the description of the invention which follows.